Test strip qualification system

ABSTRACT

In connection with a fluidic medical diagnostic device that permits measurement of the coagulation time of blood, software, methods and associated devices for quality control are disclosed. The fluidic device preferably comprises a test strip with one end having a sample port for introducing a sample and a bladder at the other end for drawing the sample to a measurement area. A channel carries sample from the sample port to an assay measurement area and first and second control measurement areas. Preferably a stop junction, between the measurement areas and bladder, halts the sample flow for measurement. If results from measurements taken for each control fall within a predetermined zone or defined limits, the assay measurement is qualified. If not, an error is registered and the test strip is counted as unfit.

FIELD OF THE INVENTION

[0001] This invention relates to approaches for qualifying resultsobtained in using analyte test strips. The invention is particularlysuited for testing the quality of test strips used for measuringprothrombin time (PT time) with whole blood in which a measurement areaincludes a composition that catalyzes the blood clotting cascade.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] FIGS. 1-3 represent information known in the art and arereferenced in the Background of the Invention. FIGS. 4 and 5diagrammatically illustrate aspects of the present invention. Variationof the invention from that shown in the figures is contemplated.

[0003]FIG. 1A is a top view of a test strip as may be used in connectionwith the present invention; FIG. 1B is a side view of that test strip.

[0004]FIG. 2A is a schematic of hardware elements for a meter for thatmay be used in the present invention; FIG. 2B shows an alternativevariation of an element of the meter in FIG. 2A.

[0005]FIG. 3 is a graph of data as used to determine PT time.

[0006]FIG. 4 is a graph showing a qualification zone for a firstcontrol.

[0007]FIG. 5 is a graph showing a qualification zone for a secondcontrol.

BACKGROUND OF THE INVENTION

[0008] European patent application EP 0 974 840 the ('840 publication),published Jan. 26, 2001, describes a device and system that may be usedwith the present invention. FIG. 1 presented herein as adapted from the840 publication shows a parallel multi-channel test strip 2. In it,measurement areas 4, 6 and 8 are provided. Upon introducing a sample,usually whole blood, at introduction port 10 and depressing a bladder 12and releasing it, a partial vacuum draws the blood though channel 14 upto shared stop junction 16. The test strip also includes a bypasschannel 18 which draws sample toward bladder 12 to alleviate negativepressure at the stop junction order to prevent overcoming the surfacetension that pins the fluid in the measurement areas at the stopjunction.

[0009] For PT measurements, it is important to stop the flow of sampleas it reaches that point to permit reproducible “rouleaux formation”—thestacking of red blood cells which is an important step in monitoringblood clotting using the present invention. The principle of stopjunction operation is described in U.S. Pat. No. 5,230,866.

[0010] A test strip body is described as preferably produced from threelayers. The elements above are formed by cutouts in intermediate layer20, sandwiched between a top layer 22 and bottom layer 24. Preferably,layer 22 is double-sided adhesive tape.

[0011] Stop junction 16 is preferably formed by an additional cutout inlayer 22 and/or 24, aligned with the cutout in layer 22 and sealed withsealing layer 26 and/or 28.

[0012] Each cutout for stop junction 16 is preferably at least as wideas channel 14. A filter may optionally be used to cover sample port 10.The filter separates red blood cells from a whole blood sample and/ormay contain a reagent to interact with the blood to provide additionalinformation. A suitable filter comprises an anisotropic membrane,preferably a polysulfone membrane of the type available from SpectralDiagnostics, Inc., (Toronto, Canada). An optional reflector may be on,or adjacent to, a surface or layer of test strip 2 and positioned overthe measurement areas. If a reflector is present, the device becomes atransflectance device.

[0013] Typically, in producing the test strip, reagent is bubble-jetprinted onto areas 4, 6 and 8. The chemicals at each site are disclosedin the 840 publication as: 1) thromboplastin in area 4; 2)thromboplastin bovine eluate, and recombinant Factor VIIa in area 6 and3) thromboplastin and bovine eluate alone in area 8. The composition inarea 6 is selected to normalize the clotting time of a blood sample bycounteracting the effect of an anticoagulant, such as warfarin. Thecomposition in area 8 is selected to partially overcome the effect of ananticoagulent. The bovine eluate (plasma barium citrate bovine eluate)is available from Haemotologic Technologies, (Burlington, Vt.);recombinant Factor VIIa from American Diagnostica, (Greenwich, Conn.).Thromboplastin (recombinant Tissue Factor/PT reagent), from OrthoClinical Diagnostics, (Raritan, N.J.).

[0014] After printing, a sample port is cut in untreated polyester filmsuch as AR1235, available from Adhesives Research, (Glen Rock, Pa.) andthen laminated, in register, to the top of the double-sided tape afterremoving the release layer. A die then cuts the stop junction throughthe three layers of the sandwich. Finally, strips of single-sidedadhesive tape such as MSX4841, available from 3M, (St. Paul, Minn.) areapplied to the outside of the polyester layers to seal the stopjunction.

[0015] Use of the test strip can be understood with reference to aschematic of the elements of a meter shown in FIGS. 2A and 2B (alsoadapted from the 840 publication), which contemplates an automatedmeter. Alternatively, manual operation is also possible. In that case,bladder 12 is manually depressed before sample is applied to port 10,then released. The first step the user performs is to turn on the meter,thereby energizing strip detector 30, sample detector 32, measurementsystem 34, and optional heater 36. The second step is to insert thestrip. Preferably, the strip is not transparent over at least a part ofits area, so that an inserted strip will block the illumination by LED38 of detector 40. (More preferably, the intermediate layer is formed ofa non-transparent material, so that background light does not entermeasurement system 34.) Detector 40 thereby senses that a strip has beeninserted and triggers bladder actuator 42 to compress bladder 12. Ameter display 44 then directs the user to apply a sample to sample port10 as the third and last step the user must perform to initiate themeasurement sequence. The empty sample port is reflective. When a sampleis introduced into the sample port, it absorbs light from LED 46 andthereby reduces the light that is reflected to detector 48. Thatreduction in light, in turn, signals actuator 42 to release bladder 12.The resultant suction in channel 14 draws sample through the measurementareas to the stop junction. For each measurement area 4, 6 and 8, a LED50 and detector 52 pair is provided to monitor the light transmittedthrough the sample as it is clotting.

[0016] Analysis of the transmitted light as a function of time (asdescribed below) permits a calculation of the PT time, which isdisplayed on the meter display 44. Preferably, sample temperature ismaintained at about 37° C. by heater 36. Each such function iscontrolled by a microprocessor chip 54 controlled by software stored inprogrammable, read-only memory or hard-wired logic 56.

[0017] As described above, the detector senses a sample in sample port10, simply by detecting a reduction in (specular) reflection of a lightsignal that is emitted by 46 and detected by 48. However, that simplesystem cannot easily distinguish between a whole blood sample and someother liquid (e.g,. blood serum) placed in the sample port in error or,even, an object (e.g., a finger) that can approach sample port 10 andcause the system to erroneously conclude that a proper sample has beenapplied.

[0018] To avoid this type of error, another embodiment measures diffusereflection from the sample port. This embodiment appears in FIG. 2B,which shows detector 48 positioned normal to the plane of strip 2. Withthe arrangement shown here, if a whole blood sample has been applied tosample port 10, the signal detected by 48 increases abruptly, because ofscattering in the blood sample, then decreases, because of rouleauxformation. The detector system 32 is thus programmed to require thattype of signal before causing actuator 42 to release bladder 12. Thedelay of several seconds in releasing the bladder does not substantiallyaffect the readings described below.

[0019]FIG. 3 depicts a typical “clot signature” curve in which currentfrom detector 50 is plotted as a function of time. Blood is firstdetected in a measurement area at time 1. In the time interval A,between points 1 and 2, the blood fills the measurement area. Thereduction in current during that time interval is due to light scatteredby red cells and is thus an approximate measure of the hematocrit. Atpoint 2, sample has filled the measurement area and is at rest, itsmovement having been stopped by the stop junction. The red cells beginto stack up like coins (rouleaux formation). The rouleaux effect allowsincreasing light transmission through the sample (and less scattering)in the time interval between points 2 and 3. At point 3, clot formationends rouleaux formation and transmission through the sample reaches amaximum. The PT time can be calculated from the interval B betweenpoints 1 and 3 or between 2 and 3. The result is typically reported interms of its “INR” (i.e., International Normalized Ratio). Thereafter,the blood changes state from liquid to a semi-solid gel, with acorresponding reduction in light transmission. The reduction in current(C) between the maximum 3 and endpoint 4 correlates with fibrinogen inthe sample.

[0020] Measurements made on a whole blood sample using the strip yield acurve of the type shown in FIG. 3 for each of the measurement areas. Thedata from the curves for the controls (measurement areas 6 and 8) areused to qualify the data from the curve for measurement area 4. Themeasurement of sample from area 4 is validated only when measurements onareas 6 and 8 yield results within a predetermined range. If either orboth of these control measurements are outside the range, then a retestwith another test strip is indicated. Such an indication would be inorder if a test strip is faulty. Ageing or oxidization of reagents canpotentially yield failing Control 1 and/or Control 2 tests.

SUMMARY OF THE INVENTION

[0021] It has been discovered, however, that in some instances teststrip readings outside a typical therapeutic range (2.0 to 8.0 INR) maynot be indicative of faulty test strips, but rather due to thecharacteristics of the test subject's blood. With earlier systems suchas that described above which indicate a test strip is faulty and ordera retest under such conditions, user compliance would merely result inanother error reading, prompting another retest.

[0022] The present invention solves a yet heretofore unknown problemwith test strip qualification accuracy outside the usual diagnostic ortherapeutic range. It may be employed to qualify formerly “falsenegative” test readings as alluded to above. Correcting such inaccuracyoffers benefits in avoiding unwarranted frustration as well as lost timeand energy—certainly for the user and possibly that of physicians whomight ultimately be consulted. While avoiding such occurrences mayindirectly result in economic savings, direct savings may be achieved bythe avoidance of erroneously disqualifying a statistically significantnumber of test strips.

[0023] Another improvement optionally offered by the present inventionis to disqualify formerly acceptable test strip readings for low INRvalues. Correcting this inaccuracy and disqualifying former “falsepositive” offers improved test strip accuracy—a benefit of clearutility. Additional benefits and advantages may also be apparent tothose with skill in the art reviewing the subject disclosure.

[0024] Systems of the present invention preferably operate in connectionwith a disposable test strips and hand held meters as described above.Mathematical algorithms or functions, preferably those described indetail below, take assay PT time into account for qualifying PT timeresults by comparison with results from one or two control-typereactions. Those algorithms as implemented by software and hardware aswell as the methodology disclosed form aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] In describing the invention in greater detail than provided inthe Summary above, the subject test strip qualification system andmethods for its use are described in relation to FIGS. 4 and 5 andvarious equations. Before the present invention is described in suchdetail, however, it is to be understood that this invention is notlimited to particular variations set forth and may, of course, vary.Various changes may be made to the invention described and equivalentsmay be substituted without departing from the true spirit and scope ofthe invention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims made herein. Furthermore, where a range of values isprovided, it is understood that every intervening value, between theupper and lower limit of that range and any other stated or interveningvalue in that stated range is encompassed within the invention. Theupper and lower limits of these smaller ranges may independently beincluded in the smaller ranges and is also encompassed within theinvention, subject to any specifically excluded limit in the statedrange. Where the stated range includes one or both of the limits, rangesexcluding either both of those included limits are also included in theinvention. Also, it is contemplated that any optional feature of theinventive variations described herein may be set forth and claimedindependently, or in combination with any one or more of the featuresdescribed herein.

[0026] Unless defined otherwise, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this invention belongs. Although any methodsand materials similar or equivalent to those described herein can alsobe used in the practice or testing of the present invention, thepreferred methods and materials are described. All existing subjectmatter mentioned herein (e.g., publications, patents, patentapplications and hardware) is incorporated by reference herein in itsentirety. The referenced items are provided solely for their disclosureprior to the filing date of the present application. Nothing herein isto be construed as an admission that the present invention is notentitled to antedate such material by virtue of prior invention.

[0027] Also, it is noted that as used herein and in the appended claims,the singular forms “a”, “and,” “said” and “the” include plural referentsunless the context clearly dictates otherwise. Conversely, it iscontemplated that the claims may be so-drafted to require singularelements or exclude any optional element indicated to be so here in thetext or drawings. This statement is intended to serve as antecedentbasis for use of such exclusive terminology as “solely,” “only” and thelike in connection with the recitation of claim elements or the use of a“negative” claim limitation(s).

[0028] Turning now to FIGS. 4 and 5, graphs representing the inventiveapproaches to test trip qualification are shown. In each graph, subjectmatter according to the present invention is shown in solid line.Differences between the present approach and another approach,previously developed by LifeScan (Milpitas, Calif.), as described inU.S. patent application titled “Test Strip Qualification System”Attorney Docket No. LIFE-044, filed on even date herewith are shown bydashed lines. The differences between each approach are indicated byhatched areas “D” and “E.” Still, each approach is preferably practicedin connection with the test strip described in reference to the '840publication.

[0029] In qualifying test strips according to either method,measurements are preferably made on whole blood sample at each of thethree test strip measurement areas, yielding curves of the type shown inFIG. 3 used to determine an INR value for each well. First, whole bloodsample is drawn into each of the reaction areas so that the fluidrehydrates the dried reagents and reacts at each site. The data obtainedfor control wells 6 and 8 are used to qualify the data from the curvefrom measurement area 4 providing PT time. The test results, includingthat for the controls, is preferably converted to INR results for use inthe algorithms described below and reporting results to the user.

[0030] Measurement areas 4, 6, and 8 preferably include suchcompositions as indicated above in connection with the '840 publicationwhere the anticoagulant in sample blood is Coumadin. Of course, whichmeasurement area includes a given composition may be varied, as may theoverall test strip configuration. In addition, variation in thereactants themselves is contemplated. Such variation in reactantchemistry would affect the results obtained and precise mathematicalrelationships described below in a predictably fashion.

[0031] As alluded to, for a PT result from the assay measurement to beconsidered reliable, it must satisfy first and second control conditions(“C1” and “C2” conditions, respectively) by falling within apredetermined result range. FIG. 4 illustrates C1 boundary conditions.Its character is determined by the compounds in reaction area 6 whichpreferably include recombinant tissue factor with buffers andpreservatives, bovine coagulation factors of the extrinsic pathway, andrecombinant factor VIIa protein. FIG. 5 illustrates C2 boundaryconditions. Its character is determined by the compounds in reactionarea 8 which preferably includes recombinant tissue factor with buffersand preservatives and bovine coagulation factors of the extrinsicpathway.

[0032] In each figure, the graphs illustrate conditions up to the usualtherapeutic value of 8.0 INR. Still, extension of the qualification zoneconditions beyond this point is possible. However, qualification forassay PT INR values in a range of 0.8 to 8.0 INR is contemplated. Anyresult of greater than 8.0 INR is considered and preferably reported asHIGH and any result less than 0.8 INR is considered and preferablyreported as LOW.

[0033] In FIG. 4, lower limit 58 and upper limit 60 for C1 INR readingsare shown. The lower limit is set at about 0.60 INR. This limit isindependent of assay INR. However, only a portion of the upper limit isindependent of assay INR. At or above an assay INR of about 2.0, thevalue of the upper limit for C1 INR is about 1.9. For lower assay INRvalues, a function dependent on assay INR determines the acceptable C1INR values. For the sake of simplicity and ease of implementation, thefunction is preferably a line equation. When expressed in the formy=mx+b, where y is C1 INR value and x is assay INR value, to best fittest data generated m (the line slope) is z≈0.50 and b (the y-intercept)is ≈0.91. By use of the “≈” sign, it is meant equals or is about equal.

[0034] This approach to qualifying PT results with C1 data differs intwo significant respects from the Test Strip Qualification Systemapplication referenced above. There, if C1 is equal to or between 0.60and 1.91 INR, the test strip is qualified so far as C1 is concerned.Such an approach is indicated in FIG. 4 where the dashed line 66continues from line 62. The hatched area D gives an indication of theimprovement in test strip qualification accuracy offered by presentinvention over the previous approach that does not account for assay INRwith respect to C1. Test accuracy is improved with respect to C1 bydiscarding low assay INR results in region D that would otherwisequalify in the above-reference method.

[0035] In FIG. 5, upper 68 and lower 70 limits for C2 INR readings areshown. As with a portion of the upper limit in C1, the upper limit 68for C2 is defined by a function dependent on assay INR value. Bytesting, C2 INR values have been observed to be proportional to assayINR values. While such a relationship may be expressed in various way,for the sake of simplicity and ease of implementation, the functiondefining upper limit 68 is preferably a line equation. When expressed inthe form y=mx+b, where y is C2 INR value, and x is assay INR value,m≈0.56 and b≈0.60 provide an excellent fit to test data generated.

[0036] Lower limit 70 is also defined by a function dependent on assayINR value. It preferably employs two line segments 72 and 74. The firstline segment 72 is coincident with the C2 lower limit line in theabove-referenced patent application. For each, m≈0.36 and b≈0.37.However, according to the referenced process, the entire lower limit isdictated by that line. This approach is shown in connection with dashedline segment 76 extending from line segment 72.

[0037] In contrast, the present invention qualifies lower value C2 INRreadings in instances where the assay INR is at or above about 4.0. Adetermination of which additional values qualify may be made bycomparison to a line segment 74 having a lesser slope, particularlywhere m≈0.15 and b≈1.2. As this function diverges from or drops-off fromline 72/76, it defines area E that results in the qualification ofadditional test strips—thus avoiding the problem of “false negatives”described above.

[0038] In actuality, the line equations described above may been definedwith greater precision. Two significant figures are expressed in orderto indicate that variation on such an order is contemplated. Still,FIGS. 4 and 5 are drawn with the precision to which the presentinvention is preferably practiced. This being said, substantialvariability in approach is contemplated as part of the presentinvention. For instance, one or more polynomial equations may be used toset C1 and C2 bounds. Alternately, tabular data representing resultswithin each qualification range or zone 78 and 80 for C1 and C2, may beemployed.

[0039] Irrespective of such changes as may be apparent to these withskill in the art, the nature or general approach of the presentinvention should not change. With respect to C1, the upper limit willtake the results of the PT assay into account for lower INR values todisqualify false positives in comparison to the referenced Test StripQualification method. With respect to C2, the lower limit will compriseat least two sections, with the second section expanding thequalification zone for a higher INR values as compared to the previousapproach.

[0040] The methodology of the present invention may be practiced witheither one or with both of these improvements. As set forth above, theimprovement in FIG. 4 (diagrammatically illustrated by area D)disqualifies formerly erroneously-indicated acceptable results; theimprovement in FIG. 5 (diagrammatically illustrated by area E) acceptsformerly erroneously-indicated negative results. Practiced together, theimprovements of the present invention offer optimal results in terms ofeconomy and accuracy in qualifying test strips.

[0041] However the present invention is implemented, in instances whereC1 and C2 results are qualified, test strip meter display 44 shows PTtime for the assay (preferably in terms of an INR value). If either orboth of these control measurements are outside the ranges defined,another sort of message indicating test reliability or fitness isdisplayed by the test strip meter. Error messages specific to the typeof failure may be presented (i.e., messages indicative of C1, C2 or C1and C2 failure). Alternately, a retest with another test strip maysimply be indicated.

Though the invention has been described in reference to a singleillustrated example, optionally incorporating various features, theinvention is not to be limited to what is described or indicated ascontemplated with respect to possible variation. The breadth of thepresent invention is to be limited only by the literal or equitablescope of the following claims. That being said, I claim:
 1. A method oftest strip qualification, said method comprising: providing a test stripcomprising an assay reaction area, a first control reaction area and asecond control reaction area; obtaining PT results for each reactionarea; comparing results from said first control area to first controlqualification criteria and results from second control area to secondcontrol qualification criteria, wherein said first control qualificationcriteria comprise an upper limit and a lower limit, said upper limitbeing at least partially dependent upon assay reaction area PT results;and outputting a message to a user indicating test strip reliability. 2.The method of claim 1, wherein said upper limit is dependent upon assayreaction area PT results at or below about 2.0 INR.
 3. The method ofclaim 2, wherein said upper limit comprises a linear function dependentupon assay reaction area PT results.
 4. The method of claim 3, whereinsaid upper limit further comprises a value independent of assay reactionarea PT result for reaction area PT results at or above about 2.0 INR.5. The method of claim 3, wherein said lower limit comprises a valueindependent of assay reaction PT result.
 6. The method of claim 1,wherein PT results obtained for each reaction area are INR values. 7.The method of claim 1, wherein said second control qualificationcriteria comprises an upper limit and a lower limit, said upper limitbeing dependent upon assay reaction area PT results, said lower limithaving first and second sections dependent upon assay reaction area PTresults, wherein said second section drops-off from said first section.8. A method of test strip qualification, said method comprising:providing a test strip comprising an assay reaction area, a firstcontrol reaction area and a second control reaction area; obtaining PTresults for each reaction area; comparing results from said firstcontrol area to first control qualification criteria and results fromsecond control area to second control qualification criteria, whereinsaid second control qualification criteria comprise an upper limit and alower limit, said upper limit being dependent upon assay reaction areaPT results, said lower limit having first and second sections dependentupon assay reaction area PT results, wherein said second sectiondrops-off from said first section; and outputting a message to a userindicating test strip reliability.
 9. The method of claim 8, whereinsaid lower limit first and second sections comprise linear functions.10. The method of claim 8, wherein said first section and said secondsection coincide at an assay reaction area PT result of about 4.0 INR.11. The method of claim 8, wherein said upper limit comprises a linearfunction.
 12. The method of claim 8, wherein PT results obtained foreach reaction area are INR values.
 13. The method of claim 8, whereinsaid first control qualification criteria comprises an upper limit and alower limit, said upper limit being at least partially dependent uponassay reaction area PT results.
 14. A method of test strip qualificationcomprising: providing a test strip comprising an assay reaction area, afirst control reaction area and a second control reaction area;obtaining PT results for each reaction area; and comparing test stripresults from said first control reaction area to a first set of criteriasubstantially as represented in FIG. 4 and comparing results from saidsecond control reaction area to a second set of criteria.
 15. The methodof claim 14, wherein said second set of criteria are substantially asrepresented in FIG.
 5. 16. A method of test strip qualificationcomprising: providing a test strip comprising an assay reaction area, afirst control reaction area and a second control reaction area;obtaining PT results for each reaction area; and comparing test stripresults from said first control reaction area to a first set of criteriaand comparing results from said second control reaction area to a secondset of criteria substantially as represented in FIG.
 5. 17. A systemprogrammed to operate according to a method selected from a group ofmethods consisting of the test strip qualification methods of claims1-16.
 18. The system of claim 17, further comprising a test stripcomprising an assay reaction area, a first control reaction area and asecond control reaction area.
 19. A computer-readable medium embodying aprogram to direct a system to perform a method selected from a group ofmethods consisting of the test strip qualification methods of claims1-16.
 20. A computer-readable medium containing data representing sampleresults, wherein said data is made by a method selected from a group ofmethods consisting of the test strip qualification methods of claims1-16.